Many electronic systems employ controllers and memory devices that send information back and forth among each other. Typically, the information is transmitted on one or more system buses. These buses act as transmission lines. Consequently, these buses require designs that consider signal reflections associated with devices coupled to a transmission line. Typically, transmission lines are terminated using a resistor coupled between the transmission line and a power supply node.
For electronic systems, such as computers, termination is provided by external resistors that are often located on the motherboard of the computer. An external resistor having an impedance that matches the impedance of the transmission line is selected to terminate a transmission line such as the interconnection signal lines of buses connecting a plurality of integrate circuits. When the external resistance matches the resistive component of the transmission line impedance, there is little or no signal reflection. However, external resistors for all the signal lines located on system boards use a large amount of area on these boards.
As an alternative to external resistors, on chip termination or on die termination, also known as active termination, can be used on the integrated circuits of the system. Using on chip termination requires additional interconnection between the devices of the system such as the controllers and the memories. This additional interconnection also requires that the controllers and other devices of the system use additional pin connectors. The amount of additional connection lines and pin connections on the various devices depends on the overall design for providing on chip termination.
What is needed is a means of providing control of active termination control that is flexible and does not require the addition of a larger number of pin connections for an electronic system.